We have measured UV endonuclease activity in 9 different lines of human cells including 4 different Xeroderma Pigmentosum (XP) lines from different genetic complementation groups. Primary skin fibroblasts and XP cells all had the same activity. HeLa cells and WI-38 cells had increased activity. We have characterized an activity in HeLa cell extracts directed against apurinic sites on DNA. These apurinic sites have been generated either by heating at low pH or as the result of alkylating agents. We have devised a technique to determine whether, in addition to apurinic sites, there is activity against alkylated bases. We have begun the purification of UV endonuclease from HeLa cells, and have achieved an initial 10-20 fold purification using ion exchange chromatography.